Microblogging site Tumblr is in the midst of a massive phishing campaign that researchers believe has successfully harvested thousands of user credentials

Tumblr is dealing with an aggressive phishing campaign. Only recently launched, it aims to steal log-in credentials and employs many of the well-known social engineering tactics that scammers use on social media sites such as Facebook and Twitter.

"Not long after this, the additional domains started to appear and then the full scale phish invasion took things up a level, with compromised accounts serving up a mixture of Tumblr hosted text and login credential submission forms served up by free webhosting accounts," researchers noted in a blog post. "While many of the compromised Tumblr accounts wanted you to login on the same page, many more besides were redirecting end-users to the tumblrlogin(dot)com website."

The ruse now making the rounds now uses another tried-and-true social engineering tactic and promises users a glimpse of pornographic content once they "revalidate" their credentials by entering them into a fake log in page. The researchers said several domains are involved in the scam, including tumblriq(dot)com, tumblrlogin(dot)com and tumblrsecurity(dot)com.

"The problem has become so pervasive that regular Tumblr users are setting up dedicated anti phishing sites to advise users of the problem. One of these sites actually pointed us in the direction of one of the dropzones used for the stolen logins, and the problem does indeed seem to be out of control at this point."

"Even accounting for the inevitable duplicates and fake data that's still quite the goldmine of pilfered login credentials," they note."What does somebody want with that many Tumblr logins? We can only guess. The stolen accounts could be used as some form of advert affiliate money making scam, or maybe we could see lots of pages with survey popups pasted over them. There is the very real possibility that the Tumblr accounts are simply a way to test if those users are logging into other services with the same credentials - at that point, everything from email accounts to internet banking sites could be fair game."